Fix MCP tool call arguments being stripped from SDK requests

[Copilot]

Problem

MCP gateway was passing empty arguments {} to backend servers, causing tool calls to fail with "missing required parameter" errors. Gateway logs showed args={} even when clients sent populated arguments.

Root Cause

The SDK handler callback signature provides arguments in two places:

func(ctx context.Context, req *sdk.CallToolRequest, args interface{})

Code was using the args parameter (SDK internal state), not req.Params.Arguments (actual client arguments as json.RawMessage).

Changes

Argument Extraction (internal/server/unified.go)

• Extract arguments from req.Params.Arguments and unmarshal to map[string]interface{}
• Default to empty map if arguments field missing or nil
• Applied to both regular tool handlers and sys_init handler

Protocol Compliance (internal/mcp/connection.go)

• Added nil check in callTool() to ensure arguments never nil for SDK-based transports
• Added ensureToolCallArguments() helper to guarantee arguments field exists for plain JSON transport
• MCP protocol requires arguments field always present, even if empty

Test Coverage

• Added connection_arguments_test.go - verifies argument handling for empty, null, and omitted cases
• Added tool_call_arguments_test.go - end-to-end verification that arguments reach backend correctly

Example

Before:

[INFO] MCP tool call request, tool=list_code_scanning_alerts, args={}
[DEBUG] github→tools/call {"params":{"arguments":{},"name":"list_code_scanning_alerts"}}
[ERROR] Backend error: missing required parameter: owner

After:

[INFO] MCP tool call request, tool=list_code_scanning_alerts, args={"owner":"org","repo":"repo"}
[DEBUG] github→tools/call {"params":{"arguments":{"owner":"org","repo":"repo"},"name":"list_code_scanning_alerts"}}
[INFO] Tool call successful

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